Recently, as electric and electronic devices have become smaller and more dense, many techniques have been employed wherein a circuit board having a plurality of components is modularized as a package for each functional block, and the necessary modules are combined so as to obtain a predetermined electrical circuit, as a substitute for a prior technique wherein individual components are mounted on a board so as to form an electrical circuit. This module is generally formed by mounting necessary components on one or both surfaces of a daughter board. However, when a technique which includes mounting components on a surface of a board is employed, a surface area of the module cannot be smaller than those of the mounted components (that is, the total foot prints of the components). For this reason, there is a limitation to high-density assembly, even when this technique is employed. Further, since the components are disposed on a flat surface according to the technique, a connection distance between the components is inevitably longer depending on its constitution, which results in a problem of an increase in loss and an increase in inductance at a high frequency.
In order to eliminate or alleviate such a problem, a module is proposed wherein components are arranged not only two-dimensionally by being mounted on a surface of a board, but also three-dimensionally by being disposed inside the board. See Japanese Patent Kokai (Laid-Open) Publication No. 11(1999)-220262(A). This publication discloses a module with a built-in circuit component which includes an electrically insulating substrate formed of a mixture comprising 70 wt % to 95 wt % of an inorganic filler and a thermosetting resin, a plurality of wiring patterns formed on at least a principal plane of the electrically insulating substrate, at least one active and/or passive component arranged in an internal portion of the electrically insulating substrate and electrically connected to the wiring patterns, and an inner via formed in the electrically insulating substrate for electrically connecting the wiring patterns. By constituting such a module, high-density can be achieved by three-dimensional connection, and the loss and inductance can be reduced by a shortened wiring length.
A capacitor is included in main components for constituting the functional module. Recently, as electronic equipment has become increasingly digitalized and operates at higher speeds, it is strongly required that the capacitor used therefor has a large capacitance and a low impedance.
Conventionally, as the capacitors, an electrolytic capacitor in which a valve metal such as aluminum or tantalum is used, and a multilayer ceramic capacitor in which Ag/Pd or Ni is used for electrodes and barium titanate is used as a dielectric, have been employed. In addition to these capacitors, a solid electrolytic capacitor in which a cathode is made of an electrically conductive polymer has been used. The solid electrolytic capacitor is preferably used, since it has a large capacitance per unit volume and it is of a thin thickness whereby the height of the module can be reduced.
A configuration of the solid electrolytic capacitor is described below. The solid electrolytic capacitor contains a capacitor element which includes a valve metal element for an anode (herein referred to anode valve metal element), a dielectric oxide film formed on a surface of the anode valve metal element, a solid electrolyte layer formed on the dielectric oxide film, and a charge collecting element for a cathode formed on the dielectric oxide film. The anode valve metal element is, for example, an aluminum foil for an anode. This foil for an anode is usually subjected to a surface roughening treatment and the dielectric oxide film is formed on the treated surface. An electrically conductive polymer layer made of polypyrrole, polythiophene, or polyaniline is formed as the solid electrolyte layer. Further, a carbon layer and an Ag paste layer are formed in the stated order, so that the charge collecting element for a cathode is formed. An anode terminal and a cathode terminal of lead frames are generally connected to this capacitor element. Further, the capacitor element is sealed with a molded resin, which results in the capacitor as the component. See Japanese Patent Kokai (Laid-Open) Publication No. 2002-198264(A).
For such a solid electrolytic capacitor, various attempts have been made to reduce an equivalent series resistance (hereinafter referred to as ESR) of the capacitor, and a low equivalent series inductance (hereinafter referred to as ESL) which is due to an external connection terminal of the capacitor. In order to reduce the ESR, a material for the electrically conductive polymer and materials for the carbon layer and the Ag paste layer have been developed. On the other hand, the anode connection is made by, for example, welding the anode to the lead flame. Therefore, the connection resistance of the anode connection is lower than that of the cathode connection. For this reason, improving the anode connection is not employed so often as a technique for reducing the ESR.
In a case where the capacitor is embedded in the substrate, as the capacitor is smaller in size, an advantage in terms of configuration such as miniaturization and higher-density of the module and an electrical advantage such as a shortened wiring length and a low impedance are obtained more effectively. However, the capacitor of the above-described configuration tends to be larger in size, since the molding resin and lead frames are disposed around the capacitor element. For this reason, such a capacitor package does not offer these advantages sufficiently. Therefore, an attempt to connect the capacitor element three-dimensionally to the board has been made by embedding the capacitor element directly in the board without using the molding resin and the lead frames.
When the capacitor is mounted on a wiring layer of a predetermined wiring pattern with solder, solder mounting which is conventionally employed for mounting a chip component cannot be employed since the valve metal element for an anode is not wetted by the solder. Further, use of lead (Pb) is restricting from the viewpoint of environmental protection, and therefore, Sn—Pb eutectic solder which has been conventionally used is nearing prohibition. As an alternative to this, a solder material which does not contain Pb is developing. The melting point of the Pb-free material is generally higher than that of the eutectic solder. As the melting point of the solder material is higher, the capacitor element is more seriously damaged by heat that is applied upon welding, which results in deterioration of the capacitor properties. In order to avoid such a disadvantage, a method for connecting the capacitor to the wiring pattern with a Pb-free electrically conductive adhesive is also employed.
However, when the anode of the capacitor and the wiring pattern are connected with the electrically conductive adhesive (herein referred to as a conductive adhesive), there is a problem of an increase in connection resistance at the anode due to the dielectric oxide film on the surface of the anode valve metal element, which makes it difficult to realize the low ESR. Further, since the surface of the valve metal element is roughened, the conductive adhesive is absorbed into pores (that is, depressed portions) of the anode formed by the surface roughening treatment. This also presents a problem of an increase in connection resistance. Further, this presents a problem of deterioration of the connection reliability because of a low bonding strength between the anode and the conductive adhesive.